Distributor Ignition Coils

A Distributor Ignition Coil multiplies battery voltage to a much higher current that can fire the spark plugs. It is at the heart of all conventional point ignition systems and most newer magnetic pick-up electronic ignition systems.

The coil contains several turns of primary winding wire which produces a magnetic field that induces a secondary voltage of thousands of times the primary current. It is this big step-up in voltage that provides the ignition spark needed to ignite the air/fuel mixture and start the combustion process.

HT (high-tension) current passes from each segment on the distributor cap, one for each cylinder, down sparkplug leads to each spark plug in the engine. This current also feeds through a central terminal in the cap which is connected to the ignition coil. At the top of the distributor shaft is a rotor arm which is driven by the camshaft in the engine and synchronized to it. The rotor is pressed against a carbon bush on the central terminal of the distributor cap. This creates a connection between the rotor and the central terminal of the distributor coil, transferring current to its outer edge. The rotor then moves in front of the internal terminal of the sparkplug and high-voltage current cuts across the gap between the sparkplug electrode and the rotor terminal and sparks it in the air-fuel mixture inside the engine.

This entire process takes place in a matter of milliseconds and occurs 18,000 times at 90 miles per hour. The purpose of the distributor is to transfer the high-voltage spark from the ignition coil to the correct spark plugs in the correct firing order and in the right amount of time. Essentially the distributor acts like a traffic cop for electricity.

A mechanical problem with this system is that the rubbing block on which the cam on the distributor shaft contacts wears continuously during use. As the rubbing block wears it changes the dwell setting, and as this happens the ignition timing may change. The correct dwell setting is critical for the optimal operation of the system.

To correct this a mechanical device is used called breaker points. These break the current going to the coil when they are pushed by the cam. When the points open, they disrupt the magnetic field in the secondary winding of the coil, which allows its primary current to collapse. The resulting high-voltage electrical pulses travel from the breaker points to the rotor arm and sparkplug lead terminals in the distributor cap. When the rotor arm and the sparkplug lead terminals make contact, a spark jumps from each spark plug terminal to the spark plug to initiate combustion and the spark is produced. The process is repeated for each cylinder in the firing order in a matter of milliseconds.